As described in Japanese Patent Publication No. 2008-170387, a known sensor device is pushed against a measurement object to detect vibrations of the measurement object. The sensor device includes a cylindrical casing and a vibration detecting unit (vibration sensor) that is housed in the casing. The vibration detecting unit is fixed with nuts with a piezoelectric element (including an electrode plate), a weight, and a rubber vibration isolator, for example, being sequentially inserted in the rear end of a detection probe. The casing houses a spring that biases the vibration detecting unit from the rear thereof to cause a tip of the detection probe to project from the casing. In this sensor device, the tip of the detection probe is brought into contact with a measurement object and the detection probe is pushed into the casing so that vibrations of the measurement object are transmitted to the detection probe and acts on the piezoelectric element as pressure variations to thereby cause voltage fluctuations. Then, a signal concerning the voltage fluctuation is sent from the electrode plate to a signal processing circuit, and vibrations of the measurement object are detected.